1. Field of the Invention
The invention relates to a bone screw for anchoring a marrow nail.
2. Description of the Prior Art
Marrow nails are used for the support of fractures at tubular bones. As a rule marrow nails are hammered into the marrow chamber in order to provide the bone with an orientation in the longitudinal direction and subsequently to effect an additional anchoring of the marrow nail in the tubular bone with screw-like anchoring elements, which are passed through transverse bores in the marrow nail.
In U.S. Pat. No. 4,622,959 a marrow nail is shown which is connected to an aiming device at its proximal end after its hammering in into the femur bone, in order to pre-bore the tubular bone in alignment with its existng transverse bores and subsequently to turn in anchoring screws which are supported with their thread at both sides by the marrow nail.
Anchoring pins and screws for marrow nails are also shown in the layering open print DE-A-32 44 243 and in the German utility model G 89 0744 3.2. Common to the listed references is that the fixing of the bone screw in its proximal region takes place with the same outer diameter as is present in the lateral thread part. In this the fixing takes place in the relatively hard corticalis of the tubular bone. A through-going screw thread which is intended to support the marrow nail with its middle region has the disadvantage that the bone loses its stretch in the proximal region of the screw due to the large screw-in length. A cylindrical prolongation of the lateral screw thread with its outer diameter into the proximal region results in a rupture action in the bone which can be countered only through an approaching of the core diameter to the outer diameter of the thread. This means that in the lateral region of the screw the thread is in such a case limited in its shaping in the proximal region by the rupture action of the cylindrical part.
The object of the present invention is to improve the fixing of a securing screw. This is achieved with the thread having a flat cylindrical thread base with a length l greater than 0.3 S and in that in the proximal head part a further thread part with the same core diameter D1 and with similar thread with the same core diameter D1 and with the same pitch S as in the distal part, however with a greater outer diameter D2 greater than D3, is provided in order to achieve a good fixing proximally and distally.
An advantage of this arrangement consists in that the geometry of the two threads can be adapted to the low yielding tendency of the corticalis. Both threads travel a short distance with in each case their own thread profile during their common penetration into the corticalis.
With an outer diameter of the proximal thread which is greater than the inner diameter of the transverse bore in the marrow nail it is ensured that the proximal part forms its own thread in the bone. In addition the thread profile is formed asymmetrically, with an inclination towards the proximal which is greater than the inclination towards the distal in order to keep the specific pressure load small, which arises on the proximally directed flank for producing a forward thrust force. Inclinations of the projecting thread profile towards the distal between 40xc2x0 and 50xc2x0 and towards the proximal between 75xc2x0 and 87xc2x0 are provided.
Notches are laid through the thread profiles so that cutting edges arise which prevent excessive radial forces and a bursting of the bone during the screwing in.
Furthermore, it is advantageous to provide the thread tips of the thread profile with a flattening of 0.1 mmxe2x89xa6axe2x89xa60.8 in the middle part and in the distal part in order that a sufficiently carrying surface is present between the transverse bore and the bone screw. A further improvement can be achieved when the proximally directed flanks at the reverse side lie on a common helix, because then a guided forward thrust movement is present independently of the momentary screw-in depth.
For the manufacture of the two thread parts with a common helix for the proximally directed flank and also in general for two different thread profiles, circular milling with two corresponding generators is useful. The profile milling tools with their axis are in each case pivoted relative to the screw axis and are guided around helically relative to the screw slug in order to mill the thread base.
Furthermore, it is advantageous to provide a puncture or a cut-out in the region proximal of the thread at the head part which prevents the core diameter of the thread from running out conically. Precisely at this location an undesirable rupture effect can arise, which produces tears in the bone.